I play Chess for 4-6 hours every single day. I play blitz seldom. I'm 54 and like to play very slow. Example: Today, like most every day, I got up at 7:00AM and played til 9:30AM. Before the rest of this day is through, I will play at least 3 more hours.

In a tournament, I don't sit for hours. I frequently get up and stroll around the room and check out the other games, occasionally step outside the playing hall to converse with a director or friend briefly. It's better for the circulation.

I see. Well my answer is the same as Estragon's. Very few people sit at the board the entire game. Get up and do a lap around the room now and then to stay fresh. Go get a refill of water/coffee, check out the standings/pairing sheet, etc.